Hepatitis B virus (HBV) is a major cause of liver disease including primary hepatocellular carcinoma and is responsible for much morbidity and mortality. Even when a universal HBV vaccine is in place the need for effective anti-HBV compounds will continue. The large reservoir of presistently infected carriers, estimated at 220 million persons, will receive no benefit from vaccination and will continue at high risk for HBV-induced liver disease. Recently two in vitro cell culture systems have been developed in which the synthesis of hepadnavirus can now be investigated. This proposal seeks to exploit both of these systems for the evaluation of compounds as antivirals for HBV. The two test systems to be used are (i) the duck hepatitis B virus (DHBV) in vitro infection of primary duck hepatocytes, and (ii) the HBV secreting cell line HepG2-T14. Two quite different classes of antivirals will be included in the study. 1. Inhibitors of the hepadnavirus reverse transcriptase/DNA polymerase. Such compounds (e.g., acyclovir) are already being implemented in treatment protocols for chronic HBV infection; others, like azidothymidine are being considered for similar use. It seems relevant and prudent that the action of such drugs be thoroughly documented and understood in the in vitro systems. 2. Nonionic methyl-phosphonate oligonucleotides. This is a new technology, not yet applied to hepadnaviruses. Specific methyl- phosphonate oligonucleotides will be synthesized and tested in the cell cultures for their ability to inhibit the synthesis of hepadnavirus by hybrid arrest of translation, reverse transcription, or morphogenesis of infectious progeny virus. Not only will these experiments help to evaluate the efficacy of both the traditional and novel antiviral compounds, but in addition they will help to dissect and document the essential details of the viral replication scheme.